DESCRIPTION: The outer edge of nearly all eukaryotic cells is rich in actin filaments. Remodeling of the actin system is an immediate consequence or a necessary feature of such events as stimulation by growth factors, activation of cell motility, transformation by oncogene products, and stimulation of secretion. Reorganization of the actin system depends on reversible associations of actin with many actin binding proteins whose activities are controlled by signals originating at membrane receptors and transduced to the cytoskeleton. Inositol lipids have recently been found to be potent regulators of several actin binding proteins, and rapid formation and hydrolysis of this class of phospholipids is a frequent event that precedes actin reorganization in many cell types. This proposal focuses on two actin binding proteins, gelsolin and profilin, that were previously characterized to be regulated in vitro by phosphoinositides and that are clearly implicated in actin cytoskeletal changes during cell stimulations that reorganize phosphoinositides. A variety of biochemical and biophysical methods will be used to defind the lipid/protein binding interfaces, and how the regulation depends on the physical state of the lipid in bilayers. A correlation to protein-phosphoinositide interactions in vivo will be made using cell-permeant peptides that selectively block gelsoline-phosphoinositide binding using platelets, neutrophils, and neuronal cells as test systems.